This invention relates to miniature telescoping spring loaded electrical probes for making electrical test connections to printed circuit boards and the like.
Electrical testing of printed circuit boards requires a large number of test connections to be made at closely spaced points on the circuit board. A matrix of closely spaced spring loaded probes projecting from an insulating board is used to make simultaneous contact with a large number of test points on the circuit board being tested. The individual probes have been constructed from an elongated conductive tubular housing with a conductive plunger telescoping inside the housing. The plunger has a contact surface projecting beyong the end of the tubular housing and a compression coil spring within the housing urges the plunger toward the end of the housing with the contact fully extended. The tubular housings of a number of probes are mounted on a common insulator base with the probes and contacts extending away from the base in closely spaced parallel relationship. By moving the base into proximity to a printed circuit under test, the probes can make simultaneous contact with a plurality of test points on the printed circuit board.
In order to make accurate and meaningful electrical measurements using the probes, it is essential that the electrical resistance introduced by the probe between the point of contact with the printed circuit board and a remote testing instrument not only be made as low as possible, but more importantly, that the resistance remain the same from one measurement to the next. In other words, the electrical resistance should remain substantially constant over the useful life of the probe, which may involve many hundreds of thousands of telescoping cycles.
In the past, performance of the probe assembly, and specifically, the resistance characteristics thorughout the life of the probe, has deteriorated due to friction particles formed by the sliding contact between the tubular housing or barrel and the plunger. The sliding action of the plunger tends to abrade the inside surface of the barrel generating particulate matter composed mostly of the base metal from which the barrel is constructed. This particulate matter is then trapped between the barrel and the plunger, the tiny particles acting to reduce the area of contact between the barrel and the plunger and causing an erratic and generally increasing level in electrical resistance during the life of the probe assembly. Also, the compression spring confined within the barrel tends to deflect laterally as it is compressed, creating a side load against the inside surface of the barrel. This side loading produces sliding contact between the coils of the spring and the inner surface of the barrel, again causing friction particles to be generated. The spring wire tends to wear due to sliding contact with the barrel, causing early fatigue failure of the compression spring.